linux-kernel - Re: [PATCH v6 3/4] rust: uaccess: add typed accessors for userspace pointers

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Date: Thu, 18 Apr 2024 13:01:20 +0000
From: Benno Lossin <benno.lossin@...ton.me>
To: Alice Ryhl <aliceryhl@...gle.com>, Miguel Ojeda <ojeda@...nel.org>, Matthew Wilcox <willy@...radead.org>, Al Viro <viro@...iv.linux.org.uk>, Andrew Morton <akpm@...ux-foundation.org>, Kees Cook <keescook@...omium.org>
Cc: Alex Gaynor <alex.gaynor@...il.com>, Wedson Almeida Filho <wedsonaf@...il.com>, Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>, Björn Roy Baron <bjorn3_gh@...tonmail.com>, Andreas Hindborg <a.hindborg@...sung.com>, Greg Kroah-Hartman <gregkh@...uxfoundation.org>, Arve Hjønnevåg <arve@...roid.com>, Todd Kjos <tkjos@...roid.com>, Martijn Coenen <maco@...roid.com>, Joel Fernandes <joel@...lfernandes.org>, Carlos Llamas <cmllamas@...gle.com>, Suren Baghdasaryan <surenb@...gle.com>, Arnd Bergmann <arnd@...db.de>, Trevor Gross <tmgross@...ch.edu>, linux-mm@...ck.org, linux-kernel@...r.kernel.org, rust-for-linux@...r.kernel.org, Christian Brauner <brauner@...nel.org>
Subject: Re: [PATCH v6 3/4] rust: uaccess: add typed accessors for userspace pointers

On 18.04.24 10:59, Alice Ryhl wrote:
> diff --git a/rust/kernel/types.rs b/rust/kernel/types.rs
> index 8fad61268465..9c57c6c75553 100644
> --- a/rust/kernel/types.rs
> +++ b/rust/kernel/types.rs
> @@ -409,3 +409,67 @@ pub enum Either<L, R> {
>      /// Constructs an instance of [`Either`] containing a value of type `R`.
>      Right(R),
>  }
> +
> +/// Types for which any bit pattern is valid.
> +///
> +/// Not all types are valid for all values. For example, a `bool` must be either zero or one, so
> +/// reading arbitrary bytes into something that contains a `bool` is not okay.
> +///
> +/// It's okay for the type to have padding, as initializing those bytes has no effect.
> +///
> +/// # Safety
> +///
> +/// All bit-patterns must be valid for this type. This type must not have interior mutability.

What is the reason for disallowing interior mutability here? I agree
that it is necessary for `AsBytes`, but I don't think we need it here.
For example it is fine to convert `u8` to `UnsafeCell<u8>`. Niches also
should not be a problem, since eg `Option<UnsafeCell<NonNull<u8>>>`
already fails the "All bit-patterns must be valid for this type".

-- 
Cheers,
Benno

> +pub unsafe trait FromBytes {}
> +
> +// SAFETY: All bit patterns are acceptable values of the types below.
> +unsafe impl FromBytes for u8 {}
> +unsafe impl FromBytes for u16 {}
> +unsafe impl FromBytes for u32 {}
> +unsafe impl FromBytes for u64 {}
> +unsafe impl FromBytes for usize {}
> +unsafe impl FromBytes for i8 {}
> +unsafe impl FromBytes for i16 {}
> +unsafe impl FromBytes for i32 {}
> +unsafe impl FromBytes for i64 {}
> +unsafe impl FromBytes for isize {}